This invention relates to detecting air inleakage into a steam turbine-condenser system and, more particularly, to a quantitative method for detecting leaks in a steam turbine system using a tracer gas.
The effective monitoring and control of air inleakage into a steam turbine-condenser system is critical to minimizing corrosion of turbine components caused by oxygen and carbon dioxide, and also to reduce vibration of low pressure turbine blading caused by air inleakage. During turbine operation, pressure within portions of the turbine is less than one atmosphere so that air may be drawn into the turbine through leaks at various joints and seals. A known method of air inleakage detection in use at the present time has disadvantages in that it fails to pinpoint the location of leaks or indicate their relative size with any degree of accuracy, because it is not quantitative. As a result, a great many man hours are often required to find leaks in a turbine-condenser system. The procedure now in use involves pulling a vacuum on the system using a condenser air exhaust pump. A calibrated flowmeter is installed at a penetration point in the turbine-condenser system just above the condenser hotwell. This creates a controllable leak which is utilized in setting leak rates. At each leak rate, an approximate amount of tracer gas, determined by a technician counting seconds, is released through a pistol nozzle across the orifice of the flowmeter from a distance of approximately three inches. Several trials are made at preset flow rates, and the average detector response of a tracer gas detector, positioned within the turbine-condenser system, is plotted against the released amounts of tracer gas, estimated semi-quantitatively, as described. During a test of the turbine-condenser system for leaks, tracer gas is released to a possible leak site outside the turbine system in an amount determined by a technician by approximating the time of release. Since the turbine system internal pressure is less than ambient, the gas is drawn into the turbine system through any seal or joint leaks. Because the time of release is not accurately measured, the amount of gas released from one test to another is variable even when the same technician is releasing the tracer gas. Leak rates are then determined from the plotted curve, using the detector responses from the tests of controlled leaks. However, leaks of the same magnitude can result in different detector responses, depending on leak location. For example, tracer gas sprayed at a leak adjacent an expansion joint between the hotwell and the condensate pump must travel through the entire turbine system before it is exhausted and detected and will cause the detector to respond with a broader, lower rise in measured tracer gas than the response generated by the same size leak in the air removal system, which latter leak causes a narrow high peak in detector response. The size and seriousness of the leaks may be substantially the same, but an instantaneous reading of the detector may not so indicate. A leak closer to the detector generally appears much worse than a leak farther away.
Typically, it takes two technicians three to five days to completely leak test a turbine-condenser system. A significant amount of turbine down time can be consumed simply waiting for the tracer gas detector to clear because too much tracer gas may have been released by the technician due to lack of quantitative controls. Moreover, it is often difficult to distinguish between major leaks, the repair of which would significantly reduce total air inleakage, and very minor leaks which could be, at least temporarily, ignored. At times, many hours are spent repairing the smaller leaks without significantly reducing overall leakage because major leaks remain, requiring additional testing and repair time. It is estimated that air inleakage testing could be reduced by one to three days by employing a more quantitative approach for measuring and determining the amount of tracer gas entering a leak site.